Labour Party leadership candidate Jeremy Corbyn arrives at a rally in Aberdeen, Scotland, late last week. Corbyn, who wants to return Britain's opposition Labor Party to its socialist roots, is the frontrunner to be elected its leader in an internal vote which began on Friday.

London — A major political party, humbled in its last attempt to win the country's highest seat, is searching for new leadership ahead of the next election. But what had originally looked like an ordinary contest between unremarkable candidates has been hijacked by a fringe contender advocating policies that the party establishment views as political suicide.

That may sound like a certain billionaire running for the US Republican presidential nomination. But it also describes the dogfight going on within Britain's Labour party, where left-wing firebrand Jeremy Corbyn is the current frontrunner for party leader.

After his party's demoralizing defeat in Britain's May 7 general election, Ed Miliband stepped down as leader of Labour, triggering an intraparty race to succeed him. Initially, the election looked like it would be a fairly routine affair between a slate of center-left figures with close ties to the party establishment.

In order to "widen the debate," several candidates helped Mr. Corbyn, a traditionalist socialist member of the “Old Labour” wing, enter the race as well. But that tactical move backfired, as Corbyn surged to the top of the polls on the back of grassroots support. All indications suggest that he will win, once all ballots are counted and announced on Sept. 12.

Q: Who is Jeremy Corbyn?

Corbyn is a former labor union official and a veteran lawmaker who first entered Parliament in 1983. More importantly, he advocates a platform unseen by Labour for more than three decades. Planks include reopening coal mines – the focus of one of Britain’s last major clashes between unions and the government – and taking some major industries, such as energy generation and transport, into state ownership. “I think we shouldn’t shy away from public participation, public investment in industry, and public control of the railways,” he said.

For supporters, Corbyn’s socialism is not only a necessary breath of fresh air, but a return to Labour’s raison d’être – and a recognition of a changed world after the 2009 financial crisis. Corbyn has never held ministerial office and cuts an unusual, somewhat austere, figure in today’s slick, business-friendly Labour party. His disconnection from the establishment is one of his main draws: as with the popularity of US Sen. Bernie Sanders (I) of Vermont, Corbyn is viewed as a candidate with clean hands, unsullied by politicking, opposed to military action abroad, and standing not only against the establishment in his own party, but for a discernible set of anti-austerity policies.

But critics, including top Labour officials, say Corbyn will destroy the party and consign it permanently to opposition.

Q: Why does Corbyn’s rise worry so many commentators and Labour lawmakers?

In the run-up to the 2015 general election, Mr. Miliband tilted left with a campaign focused on opposing cuts to state spending brought in by the Conservative-led government. The result was a resounding defeat. Now, many feel that the party must capture the center ground as it did under Tony Blair in the 1990s.

Corbyn has tilted even further left than Miliband in his policies. While party membership appears to be supportive, senior figures in the party are nervous at the prospect, with MPs whispering about refusing to participate in any shadow cabinet headed by Corbyn. Mr. Blair himself took to the pages of The Guardian newspaper to say that a Corbyn leadership “will mean rout, possibly annihilation” for Labour.

Q: Who is running against Corbyn?

The other candidates in the race are Yvette Cooper, Liz Kendall, and Andy Burnham. All three are essentially centrist figures closer to the party establishment. They all also worked extensively in politics before being elected, highlighting Corbyn’s anti-establishment position further – though he himself had a career in backroom politics before becoming an MP.

Mr. Burnham is polling closest behind Corbyn, but still trails significantly. He is standing on what he calls a “radical” platform, seeking to take newly independent schools back under government control, take control of the railways, and replace college tuition fees with a new tax on graduates.

Q: Isn’t Labour already a leftist party?

Not quite. Britain’s Labour party, founded in 1900, has always been a broad coalition of interests including union militants, moderates, Christian socialists, firebrand leftists, and even centrist figures.

And as with elsewhere in the West, there have been significant demographic shifts in Britain since the 1970s, and, consequently, in politics. Union membership has dropped from a peak of 13 million to just 6.4 million today.

The old fashioned left-right axis is also complicated by Labour's evolution to include liberal professionals. In the last election, the right-wing United Kingdom Independence Party (UKIP) surprised commentators by polling well in Labour’s working-class heartlands in the north of England. That led some to suggest Labour had become a party of middle-class “champagne socialists,” whose interests – gender issues and support for immigration – were far removed from those of Labour voters.

Q: Could a Labour pivot to the left return them to the prime minister's office?

Perhaps, but memories are still fresh of Labour's disastrous leftward swing in 1980, following Margaret Thatcher’s election as prime minister the year before.

After voting in a leftist leader, Labour put forward a manifesto for the 1983 general election calling for an EU exit, nuclear disarmament, and state ownership of industry. The party was subsequently trounced, winning 188 fewer seats than Mrs. Thatcher’s Conservatives, and found itself locked out of power until Blair won in 1997.

Corbyn’s platform is somewhat more equivocal: He has expressed skepticism about the EU, for instance writing that the EU was “a place where big business has free rein to operate,” but stopping short of advocating withdrawal. He also recently walked back plans to reinstate “Clause IV” of Labour’s constitution, which demanded state control of most industries.